This invention relates to compositions of matter and methods of using them to improve the physical properties of manufactured paper, in particular making particularly soft tissue paper. Typically, tissue paper obtains its characteristic properties of softness, bulk, absorbency, and ability to stretch, by a process involving a Yankee Dryer apparatus. In conventional tissuemaking the tissue is fed to the Yankee Dryer apparatus as a wet fiber web. The wet fiber web is largely dewatered at a pressure roll nip where the sheet is transferred to the surface of a Yankee Dryer cylinder. At this point, the paper web typically has 35-40% consistency (it is 65-60% water). The Sheet is further dried on the hot Yankee Dryer cylinder to 90-97% consistency and removed with a doctor blade. The mechanical action of the blade results in a disruption of the fiber-fiber bonds, which forms a microfold structure that gives the tissue paper its characteristic properties. This process is referred to as creping.
In order to properly crepe a paper web to make soft tissue paper, the paper web has to adhere to the surface of the Yankee dryer cylinder. The doctor blade then forms microfolds by compressing or shortening the paper web in the machine direction while at the same time separating it from the drying cylinder. This adhesion is facilitated by the application of an adhesive to the surface of the dryer cylinder. In addition, wet-end furnish components can also contribute to the adhesion that occurs. Commonly used Yankee adhesives are synthetic polymers such as polyaminoamide-epichlorohydrin resins, polyamine-epichlorohydrin resins, polyvinyl alcohols, polyvinyl acetates, polyacrylamides, polyamines, polyamides, polyvinylpyrrolidones and polyethers. Other natural and derivitized natural polymers may also be employed including starch, guar gum, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and the like. Various low molecular weight compounds, oils and surfactants, are used to modify the properties of these adhesives.
The tissue industry has a continuing interest in manufacturing premium grade tissues, which are tissues with high levels of softness. Improvements in softness can be obtained by modifying the fiber source, implementing particular forming and drying strategies, creping the fiber sheets, and by using wet-end or topical application of softening agents. Creping the paper sheet when it has a very low sheet moisture level (<3%) is one of the most effective ways of achieving desired levels of high softness. At low moisture levels, the sheet and the coating tend to adhere to each other more strongly which causes the sheet to debond in the Z-direction more efficiently thereby generating greater bulk and softness.
Despite the benefits for tissue softness, low moisture creping is not being widely practiced due to coating runnability issues. Conventional creping adhesives typically develop a hard coating which is less rewettable after undergoing the extensive drying that is required for low moisture creping. This hard coating results in a loss of adhesion and also results in blade vibration (chatter), which can cause non-uniform creping, blade wear, and, in extreme cases, damage to the Yankee dryer cylinder surface. Thus there is a great demand for a creping adhesive that remains soft and rewettable under extreme drying conditions.